1. Field of the Invention
The invention relates to mine roof bolt assemblies, and more particularly to flexible washer/gaskets for mine roof bolt assemblies for sealing the interior of mine roof bolt holes from mine air.
2. Description of the Prior Art
A variety of mine roof bolt assemblies are known in the art. They usually include a bolt of from three to six feet in length, a roof plate or support plate through which the roof bolt extends, and an expansion shell threaded onto a threaded end of the roof bolt. A mine roof bolt hole is drilled, usually perpendicularly to the surface of the mine roof, with the expansion shell inserted into the roof bolt hole such that the support plate abutts the mine roof. The roof bolt is tightened, causing the expansion shell to expand, thereby anchoring the entire assembly into the mine roof strata and forcing the support plate upwardly against the mine roof. The mine roof strata is known to be composed of various layers of different types of rock having varying strength characteristics. A plurality of spaced mine roof bolts installed in the mine roof tends to secure the various layers of mine roof strata together to prevent slippage therebetween, increasing the strength of the laminated strata, thereby preventing caving of the mine roof. However, up to now, the known mine roof bolt assemblies have not satisfactorily supported mine roofs wherein mine roof strata exposed by the roof bolt holes to ambient mine air and moisture has caused certain kinds of mine roof strata to weaken. Mine roof bolt assemblies are known to fall out of degraded roof bolt holes, eliminating the strata layer binding needed to prevent slippage between different layers of mine roof strata. It is also known that in certain cases a mine roof may collapse even though all of the mine roof bolts therein are sufficiently tightly anchored in a hard layer, such as limestone. Up to now, the cause of the later mentioned caving has not been well understood. U.S. Pat. No. 2,829,502 describes a mine roof bolt assembly for excluding mine air from a mine roof bolt hole to prevent spalling or crumbling of the side walls of the interior of the roof bolt hole by use of a large conical stopper-like washer on the shaft of a particular type of roof bolt. U.S. Pat. No. 3,651,651 shows a stabilizing bushing and a flat washer. U.S. Pat. No. 3,521,454 illustrates a flexible annular washer which accommodates variations in the surface surrounding the mouth of a rock bolt hole. However, none of the known rock bolt or roof bolt assemblies provide reliable sealing of the interior of the mine roof bolt hole from mine air in many practical instances. One instance is that the roof bolt, when tightened, or "torqued", may be anchored by expansion of expansion shell in such a way that the lower end of the roof bolt is forced toward one side of the roof bolt hole, preventing the known washers from being reliably utilized. Further, certain of the known washers are not readily adaptable to mine roof bolts and support plates which are nowadays commonly used, particularly roof bolts and corresponding support plates which permit a certain amount of swiveling of the roof bolt shaft with respect to the support plate; this is a highly desirable feature, since roof bolt holes are frequently not perfectly perpendicular to the portion of the mine roof surrounding the mouth of roof bolt holes. Yet, in order to provide adequate functioning of the roof bolt assemblies and to prevent degradation of the interior walls of the roof bolt holes, there exists a need for a reliable washer/gasket which reliably seals the interiors of roof bolt holes from mine air, even though such roof bolt holes are not perpendicular to the surrounding portions of mine roof and even though the roof bolts are misaligned with respect to the roof bolt holes as a result of torquing of the roof bolts. Other United States Patents which generally indicate the state-of-the-art are for rock bolts and mine roof bolts such as U.S. Pat. Nos. 2,950,602; Re 25,869; 2,892,650; 1,960,750; 3,693,359; 3,653,217; and 3,379,016. Another known approach to expelling air from mine roof bolts involves injection of or provision of various resin compounds to fill up roof bolt holes. However, such compounds and the means for providing or installing such compounds within roof bolt holes are unduly expensive.